Monometallic clusters

It was emphasized that cluster redox properties depended on the nuclearity, mostly at low n values. The oligomers are spontaneously unstable with respect to coalescence and the determination of the redox properties of these transient oligomers is again accessible only by means of a kinetic approach. The clusters are formed as above by using a pulse to induce atoms which then coalesce during the coalescence they can react with an added reactant. Depending on the chemical properties of the reactant and on their nuclearity n, the clusters may behave as electron acceptors or donors. 

Radiolytic reducing radicals can be used, during the same pulse, partly to form the metal atoms (Eqs. 2, 8, and 9), and partly to produce, from a precursor S, the electron donor S (Fig. 8) which will impose the potential threshold for the reaction of electron transfer to the cluster (Eqs. 29-31)  

The couple S/S is selected with a specific and intense optical absorption of S or S , so that the electron-transfer reaction can be observed directly. In the early stages of atom coalescence, the redox potentials of the atom and of the smallest clusters are generally far below that of the donor and the transfer from S to the oligomer does not occur. The ion reduction is caused exclusively by solvated electrons and alcohol radicals (Eqs. 2, 8, and 9). The nucleation and coalescence dynamics are thus the same as in the absence of (Eqs. 10 and 11). Beyond a certain critical time, tc, that is large enough to enable the growth of clusters and the increase of their potential above the threshold imposed by the electron donor S , electron transfer from this monitor to the supercritical clusters is allowed (Eq. 32) and detected by the absorbance decay of S (Fig. 6). For n ny. 

If the concentration of is high, the reactions depicted by Eqs. (29)-(31) are faster than the coalescence reactions (Eqs. 10 and 11) with a fixed total concentration of atoms and the clusters now grow mostly by successive additions of supplementary reduced atoms (electron plus ion). It has been shown that once formed, a critical cluster, of silver for example,indeed behaves as a growth nucleus. Alternate reactions of electron transfer (Eqs. 32 and 34) and adsorption of surrounding metal ions (Eq. 33) make its redox potential more and more favorable to the transfer (Fig. 8), and autocatalytic growth is observed.The observation of an effective transfer therefore implies that the potential of the critical cluster is at least slightly more positive than that of the electron-donor system, i.e. °(M +/M ) °(S/S-). 

The value of the nuclearity of this critical cluster enabling transfer from the monitor depends on the redox potential of the selected donor, S . We studied electron transfer to silver clusters from the decay of different electron donors, 

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In many cases, although and M are both readily reduced by radiolytic radicals, a further electron transfer from the more electronegative atoms (for example, M ) to the more noble ions ( °(M /M )electron transfer is also possible between the low valencies of both metals, so increasing the probability of segregation [174]. The intermetal electron transfer has been observe directly by pulse techniques for some systems [66,175,176], and the transient cluster (MM ) sometimes identified such as (AgTl) or (AgCo) [176]. The less noble metal ions act as an electron relay toward the precious metal ions, so long as all are not reduced. Thus, monometallic clusters M are formed first and M ions are reduced afterward in situ when adsorbed at the surface... 

The effect of catalyst composition on the selectivity to the desired CDE is remarkable. It is clear that monometallic cluster catalysts almost completely hydrogenate CDT to CDA, whereas all three bimetallic clusters (RuSn, RhSn, and PtSn ) exhibit high selectivity toward CDE at approximately the same CDT conversion (50-75%). Pure tin clusters derived from Ph SnH are poor catalysts both in terms of activity and selectivity (conversion less than 10%). 

While more information pertaining to the electronic and atomic structures of the bare and silica-anchored bimetallic clusters is required - and DFT calculations and X-ray absorption spectroscopy studies are underway - it is striking that the performance of the small bimetallic clusters exceeds that of the pure Pd, the preferred catalyst in the industrial process [55]. Moreover, it is especially noteworthy that monometallic cluster catalysts exhibit such poor selectivity and activity, and that the presence of Sn as a stoichiometric component of the bimetallic clusters confers such a powerful influence on the minority (PGM) component. 

Figure 4.23 X-ray diffraction patterns for a platinum-iridium bimetallic cluster catalyst and for reference materials consisting of physical mixtures of platinum and iridium in the form of large crystals or dispersed monometallic clusters (4). (Reprinted with permission from Academic Press, Inc.)...

Because the dynamics observed by means of pulse radiolysis indicated that the displacement process was not instantaneous, it was suggested that very short, intense irradiation, with a dose sufficient to achieve the complete reduction of all the ions, could efficiently prevent the segregation, due to electron transfer between the metals. Therefore, the method could enable the formation of alloyed clusters, of major interest for various applications, particularly catalysis. The positive influence of high dose rates, which quench the atoms in an alloyed cluster, has been demonstrated a bilayered cluster would be obtained from the same system by irradiation at a lower dose rate. " Moreover, as for monometallic clusters (Section 3.13.4.3), the high dose rate favors nucleation rather than growth, and the final sizes of the alloyed clusters are particularly small. " " ... 

Table 3.15. Differences in binding energy of a hydrogen atom bonded to a bimetallic cluster and to a monometallic cluster. A (bi) A (mono) (energy unit eV),...

The smallest monometallic Au clusters are the two AU4 clusters, Au4(PPh3)4l2 and Au4(dppm)3l2. Both clusters contain a tetrahedral AU4 core . In Au4(PPhj)4l2 the... 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Synthesis methods such as those described earlier for monometallics have been applied with metal carbonyls incorporating two metals. The resultant supported species may be small supported metal clusters [41,42], and, as for monometallics, the usual products are supported species that are nonuniform in both composition and structure [42]. There are several examples of well-defined metal carbonyl clusters in this category but hardly any examples of well-defined decarbonylated bimetalhcs on supports. 

Our first attempt of a successive reduction method was utilized to PVP-protected Au/Pd bimetallic nanoparticles [125]. An alcohol reduction of Pd ions in the presence of Au nanoparticles did not provide the bimetallic nanoparticles but the mixtures of distinct Au and Pd monometallic nanoparticles, while an alcohol reduction of Au ions in the presence of Pd nanoparticles can provide AuPd bimetallic nanoparticles. Unexpectedly, these bimetallic nanoparticles did not have a core/shell structure, which was obtained from a simultaneous reduction of the corresponding two metal ions. This difference in the structure may be derived from the redox potentials of Pd and Au ions. When Au ions are added in the solution of enough small Pd nanoparticles, some Pd atoms on the particles reduce the Au ions to Au atoms. The oxidized Pd ions are then reduced again by an alcohol to deposit on the particles. This process may form with the particles a cluster-in-cluster structure, and does not produce Pd-core/ Au-shell bimetallic nanoparticles. On the other hand, the formation of PVP-protected Pd-core/Ni-shell bimetallic nanoparticles proceeded by a successive alcohol reduction [126]. 

Li and Armor reported that Co-exchanged zeolites present a very high catalytic performance for the CH4-SCR, even in oxygen excess conditions [1, 3], Bimetallic Pt-and Pd-Co zeolites have revealed an increase of activity, selectivity towards N2 and stability, when compared with monometallic Co catalysts [4-8] even in the presence of water in the feed. Recent works show that these catalytic improvements are due to the presence of specific metal species as isolated metal ions, clusters and oxides and their location inside the cavities or in the external surface of zeolite crystallites [9, 10],... 

An important result of the multinuclear NMR investigations of 23— 25, 27, 28, and 31-35 is that the structures, in contrast to aggregates of monometalated secondary phosphanes and arsanes, are retained in solution. Thus, the phosphandiide derivatives here discussed show resonance signals in their 31P NMR spectra that are independent of concentration and temperature (see Table III). The 31P and 27A1 NMR chemical shifts of 23-25, 27, and 31-35 differ with respect to ring size of the clusters and electronic influences by the substituents at phosphorus and aluminum. 

Heterobimetallic complexes that induce reactions at significantly faster rates than (or notably different product selectivities from) monometallic derivatives are probably genuine cluster catalysts. 

Reaction of 252 with Co2(CO)g under conditions of high concentration of the di-cobalt carbonyl gives trimetallic (77S-CsMes)Ru (77S-CsMes)RuCO Go(CO)2(/t3-CO)B3H6, 255, whereas low concentrations allow competitive degradation to yield monometallic (77S-CsMes)Ru (CO)(/i-H)B3H7 256.153,164 This reaction also affords ((77S-CsMes)Ru)2(CO)2B3H7 257 in variable yield dependent on the solvent used, as well as the linear trimetallic cluster 314 (Section 3.04.3.3.5) in very low yield.165... 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

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